Multiconductor cables are widely used for the transmission of digital data and control signals. In general, the cables include multiple individual conductors terminating at both ends in pins or sockets that are grouped together in a connector. As digital control systems have become more common, multiconductor cables have become more varied. Modern multiconductor cables are found in a very large variety of connector shapes and sizes, number of conductors, and internal wiring schemes. Examples range from small ribbon cables used in personal computers that may have only a few conductors to massive cable harnesses used in aircraft control systems that may have hundreds or thousands of individual conductors.
As the complexity of the cable wiring schemes has increased, so too has the variety of cable testing equipment. Most conventional testing devices focus on determining the continuity paths between the individual pins of the cable. Determining the continuity paths serves two purposes: identifying the internal wiring of the cable; and detecting faults or defects in the cable. In general, conventional testing systems check continuity by applying a test signal to one or more test pins at one end of the cable and searching the remaining pins at one or both ends of the cable for the presence of the applied signal. If any of the remaining pins receives the test signal, that indicates a continuity path between that pin and the test pin.
The most sophisticated systems are capable of quickly determining complex wiring arrangements, cross-checking the continuity results against the wiring schematic stored in memory, and saving, displaying, or printing the test results for future comparison or record keeping purposes. If the signal is not received at the appropriate pins, an open circuit condition (open) is indicated, while the presence of the signal on pins where there should be no signal indicates a short circuit condition (short).
These faults can result from any number of physical defects or manufacturing errors including broken wires, incorrect pin fit, deformed pins, missing springs, cold solder joints, and bad crimps. Moreover, these defects manifest themselves in two different forms of faults: hard faults, which are permanent or long-lasting opens or shorts; and intermittent faults, which are opens or shorts that may last only microseconds or milliseconds.
Conventional systems are not capable of detecting intermittent faults, i.e. opens or shorts lasting only a few microseconds or milliseconds. Intermittent faults are generally caused by the same types of problems that lead to hard faults, but in a less severe form. For example, although a conductor may be cracked or broken at a given point, the severed ends of the conductor may usually remain in contact with one another, breaking contact only periodically, and for a very short time, constituting therefore an intermittent fault.
The only way conventional systems could detect such a short-lived fault is if the fault occurred exactly when that pin or conductor was being tested, and for long enough to be detected by the system. As a result, many cables that pass conventional continuity analysis may actually have multiple defects that lead to intermittent faults when the cable is placed into operation, particularly under harsh ambient conditions. In fact, recent information indicates that intermittent faults in multiconductor cables form one of the most common and significant sources of error in digital control systems.
Accordingly, conventional multiconductor cable systems and testing techniques must be modified to detect intermittent faults. This modification must be twofold: first, intermittent faults must be induced or generated in the cable during the testing procedure; and second, the testing system must be capable of detecting the intermittent faults when they occur.
Both modifications of the conventional system must be present to ensure accurate fault detection. For example, even if intermittent faults were being generated during testing, many conventional systems would not be capable of detecting a fault lasting only a few microseconds or milliseconds. Likewise, those systems that might be fast enough to detect intermittent faults would not be able to do so if the faults were not being induced at the precise moment when the individual pin or conductor connected to that fault were being tested.